The present invention relates to a tea bag and to a method and apparatus for making it. In particular, the invention concerns a tea bag of the type having two chambers containing comminuted tea that is made by forming a strip of filter paper into a flattened tube, cutting the tube to length, folding the tube intermediate its ends along a W-fold to form two chambers, and joining the ends of the chambers by a headfold. Tea bags of this type are well-known and have been marketed for about 40 years. For example, Thomas J. Lipton Co. sells such tea bags under the trademark "FLO-THRU.RTM.." For convenience, this type of tea bag will be referred to herein as a "double-chamber" tea bag.
Double-chamber tea bags are produced by special machines, of which there are several manufacturers. One of the main suppliers is a German firm, Teepak, GmbH, which sells a machine known as the "Constanta." The Constanta and similar machines produce double-chamber tea bags by depositing measured masses of comminuted tea in spaced apart locations along the length of a strip of filter paper at a first station. The filter paper strip is then advanced through mechanisms that form it into a flattened tube by folding in portions along each side and bringing the edges together face to face with a band along one edge of the strip extending beyond the other edge of the strip. The extending band is folded over the other edge to form a loop, and the loop is folded over, thereby forming a doubly folded joint between the edges of the strip in which one edge is singly folded over, the other edge is doubly folded and the extending band is captured in a loop of the first edge. The doubly-folded joint, which is called a "centerfold," is crimped by moving the tube between a pair of wheels having serrated surfaces in the form of transverse, projecting teeth. The wheel that engages the side of the centerfold opposite from the folded over edges has a width approximately equal to the width of the centerfold. The wheel that engages the folded edges is somewhat narrower than the centerfold. The teeth are designed to mesh, such that they tend to form corrugations in the centerfold. The narrow toothed wheel is spring-loaded toward the wide toothed wheel to apply pressure to the centerfold and cause the fibers of the several layers that constitute the centerfold to intertwine and form mechanical bonds between the adjacent layers.
One problem with presently known double-chamber tea bags is that the centerfold occasionally comes apart when the tea is being made, thereby releasing some of the tea from the bag. The failure of the centerfold, which does not happen very often but is aggravating to the consumer when it does, generally results from a sudden build-up in the pressure of air in the bag due to heating of the air when hot water is poured onto it, usually accompanied by a mechanical force resulting from impingement of a water stream on the bag.
Another problem with double-chamber tea bags involves release of tea from the front chamber of the bag at the headfold, again due to an air pressure increase and the force of a water stream. The headfold of previously known double-chamber tea bags is produced by positioning an end portion of the front chamber back to back against an end portion of the back chamber such that a band adjacent the end of the rear chamber extends beyond the end of the front chamber. Triangular segments at each corner of both chambers adjacent their ends are folded over along respective coincident diagonal fold lines such the triangular segments overlie portions of the front chamber externally. The extending band of the rear chamber is folded over along a transverse foldline to form a flap that overlies portions of the triangular segments externally. A portion of the end of the front chamber between the diagonal folded segments is not folded and is merely covered but not effectively closed by the folded over flap of the rear chamber. Tea can escape from the open portion along the end of the front chamber.